1. Field of the Invention
The present invention relates to a semiconductor film having a crystal structure formed by using a technique of laser crystallization and a semiconductor device such as a thin film transistor (hereinafter, TFT) including the semiconductor film and methods for manufacturing thereof. More specifically, the present invention relates to a semiconductor film having a specific crystal structure formed by a technique of laser crystallization, in other words, a plane orientation of crystal grains of three directions that are perpendicular to each other is aligned at equal to or more than a certain ratio. The present invention also relates to a semiconductor device including the semiconductor film, and a method for manufacturing thereof.
2. Description of the Related Art
In recent years, the technique of crystallizing an amorphous semiconductor film formed over a glass substrate by laser irradiation to form a semiconductor film having a crystal structure (hereinafter referred to as a crystalline semiconductor film) has been widely researched, and numerous suggestions are given. Since that crystalline semiconductor film has higher mobility compared to an amorphous semiconductor film, this crystalline semiconductor film is utilized for manufacturing of TFTs for, for example, an active matrix type liquid crystal display device, an organic EL display device, and the like in which over a single glass substrate, TFTs are formed for a pixel portion or for the pixel portion and a driver circuit.
As the method of crystallization, except for the laser crystallization, there are a thermal annealing using a furnace and a rapid thermal annealing (RTA); however, when the laser crystallization is used, without much raise in temperature, heat is only absorbed by the semiconductor film and crystallization is performed; therefore, material such as glass, plastic, and the like that have low melting point can be used as the substrate. As a result, a glass substrate which is inexpensive and is easily processed to have large area can be used, and production efficiency can be drastically improved by the laser crystallization; therefore the laser crystallization is an excellent technique of crystallization.
This applicant focused attention on excellent characteristics of the crystallization, eagerly strived to manufacture a semiconductor film by laser crystallization, and consequently succeeded in the development of many techniques. Among the successful techniques, there is a technique of obtaining excellent semiconductor characteristics: a non-crystallized film which has reduced concentration of oxygen, nitrogen, and carbon is irradiated with a laser light, and melting and re-crystallization steps are performed to obtain a crystalline semiconductor film having high mobility (Patent Document 1). In the suggestion of this technique, it is disclosed that at laser irradiation, it is preferable for a protective film to be formed over the non-crystallized film, and it is also disclosed that by the placement of the protective film, impurity mixing into a silicon film can be evaded.    [Patent Document 1] Japanese Published Patent Application No. H5-299339